1. Field of the Invention
The present invention pertains to remote pressure or vacuum sensing devices. More specifically, the present invention pertains to a pressure or vacuum sensing device capable of remote sensing of the positioning and operation of relief valves or other pressure and vacuum sensing devices.
2. Description of the Prior Art
Most pressure or vacuum vessels and systems are provided with some type of pressure relief device which, when pressure or vacuum within the vessel or system reaches an unsafe or undesirable level, opens to relieve or ameliorate the undesired pressure or vacuum condition. This prevents damage to the vessel or other components of a pressurized or vacuum system and in many cases may prevent life threatening, environmental or economical catastrophes.
Most relief devices are in the form of a spring loaded relief valve which opens to relieve an unwanted pressure or vacuum or a rupture disk which ruptures to relieve such a condition. Both types of relief devices have limitations and are subject to malfunctions. Spring operated relief valves are typically set and tested at a test facility prior to installation on a vessel or in a pressurized system. The valve is set to operate or relieve at a particular pressure level. Although such a relief valve may operate as intended when first installed, it may, over a period of time, cease to function as intended due to corrosion, erosion, vibration, temperature, fatigue or any number of other mechanical or environmental conditions. Furthermore, relief valves frequently open and close within mili-seconds. Opening and closing may not be uniform and many times the valve may reseat differently causing the valve to not function as intended in subsequent operation. Relief valves are also noisy, causing potential interference with other devices. In addition, it is desirable to know exactly when and how long a relief valve opens and/or closes as well as the distance traveled by the valve closure member when opened. Such operation is not easily monitored with the typical relief valve.
Rupture type relief devices also have certain disadvantages. For example, when pressure or vacuum conditions in a vessel or system reach a pre-determined level, the rupture disk or element of a rupture type relief device ruptures to relieve the pressure or vacuum condition. When this occurs, it is necessary to replace the ruptured element after the pressure or vacuum problem is corrected. In addition, rupture devices are also subject to improper functioning due to vibration, corrosion, erosion, fatigue, etc. Rupture disks require at least a specified pressure or vacuum for operation. Therefore, they are not suitable for very low pressure or vacuum conditions. Furthermore, rupture disks, of fixed form, are pre-set and cannot be adjusted for different settings.
In more recent years, particularly due to heightened environmental and safety concerns, various devices have been designed for monitoring the operation of various relief devices. For example, U.S. Pat. No. 4,342,988 discloses a system for detecting actual or impending failure of the rupture disk of a rupture type relief device. In this system, strain gauges and wires are attached to the rupture disk. When the disk ruptures the wires are severed, closing an electronic circuit and providing a remote signal to indicate rupture. One of the problems of such a system is the exposure of the relieved fluids to an electrical current which may cause an explosion. If this type of device is utilized to monitor operation of a spring type relief valve, the valve must be taken out of service to remove the device when ruptured. This results in down time, shutting down operation of the protected process while the device is sent to a shop for disk, wire and strain gauge replacement and testing.
In more recent years, a number of other pressure responsive control devices or switches have been developed which utilize diaphragm or bellow operated actuators. Examples of such may be seen in U.S. Pat. Nos. 5,198,632; 5,216,213; 5,281,782; 5,331,126; etc. In most of these designs, the diaphragm of the actuator is disposed in a cavity, dividing the cavity into a pair of chambers, one chamber being exposed to the pressurized environment and the other chamber housing or being exposed to the electrical switching apparatus. Thus, if the diaphragm ruptures, it exposes the switching circuitry to the fluids of the pressurized environment, resulting in a potentially hazardous and part damaging environment. In addition, most of these systems incorporate many complex parts, resulting in higher cost and greater susceptibility to malfunctioning. Most of them are limited in movement and do not have the ability to incorporate optic or radio frequency type sensing devices or sensing devices which require smooth elongated linear travel. Most of them are not easily and precisely adjustable.
It is therefore clear, that most of the pressure sensing devices, particularly those which utilize rupture disks, diaphragms, etc. are not totally acceptable for monitoring operation of pressure relief valves and the like. Safer, more reliable, easier to use and less expensive monitoring devices are needed.